Glycerol (also called glycerine) is produced by the methanolysis of vegetable oils at the same time as the methyl esters which are employed in particular as motor fuels or fuels in diesel and home-heating oil. It is a natural product, available in large quantities, and can be stored and transported without difficulty. It has the advantage of being a renewable raw material meeting the criteria associated with the new concept of “green chemistry”. The development of glycerol has attracted considerable research, and the preparation of acrylic acid is one of the alternatives considered.
Application WO 06/114506 describes a method for preparing acrylic acid in one step by the oxydehydration reaction of glycerol in the presence of molecular oxygen. The principle of the method is based on the two consecutive dehydration and oxidation reactions:CH2OH—CHOH—CH2OH→CH2═CH—CHO+2H2OCH2═CH—CHO+½O2→CH2═CH—COOH
The presence of oxygen serves to carry out an oxidation reaction, following the glycerol dehydration reaction, leading to the formation of acrylic acid from the glycerol in a single step. This method can be implemented in the gas phase or the liquid phase, with concentrated or dilute aqueous solutions of glycerol. This method for producing acrylic acid directly from glycerol is particularly advantageous because it allows synthesis in a single reactor. However, it is necessary to introduce all the molecular oxygen from the dehydration stage. This has many drawbacks, in particular the reaction in the first dehydration step risks running out of control by combustion, and furthermore, when the source of molecular oxygen is air, the reactor must be much larger because of the presence of nitrogen in the air.
In patent application EP 1 710 227, the reaction product resulting from the gas phase glycerol dehydration reaction is subjected to a subsequent gas phase oxidation step to obtain acrylic acid. The method is implemented in two reactors in series, each comprising a catalyst suitable for the reaction carried out. It is recommended to add oxygen to the gas mixture fed to the second reactor, in order to improve the oxidation reaction and to obtain acrylic acid with a high yield. This two-step method is implemented with pure glycerol or with aqueous solutions comprising more than 50% by weight of glycerol. It is recommended to use a concentrated glycerol solution in order to limit the energy cost associated with the evaporation of the aqueous solution and the cost incurred by wastewater treatment. However, if the glycerol concentration is too high, more undesirable reactions are liable to occur, like the formation of glycerol ethers, or reactions between the acrolein or acrylic acid produced and the glycerol.
International application WO 2006/092272 describes a method for preparing acrylic acid from glycerol comprising either a liquid-phase glycerol dehydration step, or a gas-phase dehydration step. According to example 1, the gaseous reaction mixture containing the acrolein obtained from the gas phase glycerol dehydration reaction is contacted with water in a quench unit before being sent to the oxidation reactor.
In the method for preparing acrylic acid from glycerol described in international application WO 2006/136336, the aqueous stream leaving the dehydration reactor is treated in order to recycle to the reactor an acrolein-depleted phase containing the unreacted glycerol and to supply the oxidation reactor with an acrolein-enriched phase. The dehydration reaction is carried out at high pressure, particularly at a pressure above 50 bar, using very dilute aqueous solutions of glycerol, in particular containing less than 10% by weight of glycerol.
The use of an aqueous solution of glycerol in a two-step method has the drawback of producing, at the outlet of the first stage, a stream containing not only the acrolein produced and the by-products, but also a large quantity of water, originating partly from the glycerol solution, and partly from the water produced by the dehydration reaction. This stream is sent to the second reactor, where the acrolein is oxidized to acrylic acid in the presence of a catalyst. The conventional catalysts for this oxidation reaction are generally solids containing at least one element selected from Mo, V, W, Re, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, Te, Sb, Bi, Pt, Pd, Ru, Rh, present in metal form or oxide, nitrate, carbonate, sulphate or phosphate form. Certain elements, such as molybdenum, tellurium or rhenium, are volatile, particularly in the presence of water. This means that the second stage catalyst loses its efficiency and its mechanical strength rapidly in the presence of the stream of water, making the maintenance of the method difficult. Moreover, the acrylic acid, produced in a dilute aqueous solution, requires separation and concentration steps which are generally complicated and fairly costly.
However, it has been surprisingly found that the presence of water in the dehydration reactor serves to promote the gas phase glycerol dehydration reaction by limiting the deactivation of the dehydration catalyst.